1. Field of the Invention
The invention relates to a high and medium performance tire for vehicles and, more particularly, to a tire having characteristics that are especially though not exclusively suitable for use on winding circuits and dry roads.
2. Description of the Related Art
A tire in its more general form comprises: a casing structure including a central crown portion and two axially opposite sides terminating in a pair of ribs for the attachment to the rim of a wheel; a belt structure coaxially associated with the casing structure and a tread band extended coaxially around the belt structure.
Generally the tread band comprises a raised pattern formed by a plurality of transversal and longitudinal grooves giving rise overall to a plurality of ribs distributed according to configurations of differing types, for example along a central zone straddling the equatorial plane and in at least two shoulder zones extending in positions coaxially opposite the central zone.
As the tire rolls, the ribs on the tread strip are subjected to a set of thermo-mechanical stresses, which are all the greater the more severe the conditions of use and which result in changes to their geometry and, over varying time periods, to a deterioration in the tire's performance.
In particular, the contact with the ground during running produces a flexing of the trailing edge of the ribs of the tire, said edge by drooping radially towards the inside and moving in the direction opposite the direction of running produces a restriction of the groove that separates it from the next rib. This phenomenon leaves the leading edge of the latter-named open to a tangential stress which is repeated cyclically at each revolution of the wheel, causing a typical uneven and premature wear known as “saw-tooth wear”.
Tyres that try to minimize the number of ribs and strengthen their structure are known in the sector art.
For example, the patent application WO 98/25776 refers to a tire comprising a tread band provided with transversal grooves in three distinct zones, a central zone straddling the equatorial plane and two shoulder zones, so as to define a directional type tread profile, i.e. one having a preferential direction of rotation.
Each transversal groove comprises a first stretch extending within a shoulder zone according to a direction perpendicular to the equatorial plane and a second stretch extending obliquely inside the equatorial zone.
The transversal grooves are distributed circumferentially with a pitch “p” and extend alternatively from opposite shoulder zones.
More particularly, in the abovementioned tread band the transversal grooves are distributed in groups; and more specifically, the tread band comprises a first group of three grooves, parallel to one another, repeated circumferentially with a pitch “P” on the left of the equatorial plane and alternating with a second group of three grooves, parallel to one another, and repeated with an identical pitch “P” on the right of the equatorial plane.
In each group the grooves are of decreasing length in the direction of motion of the tire, such that one of the grooves passes through the equatorial plane whereas the remaining two grooves are terminated in the vicinity of the equatorial plane, at a different distance from the latter.
The configuration of the transversal grooves is such as to produce, in the space between the alternating groups, a central rib extending circumferentially in continuously zigzag fashion and a plurality of transversal blocks that leave from the central rib and stretch obliquely right and left of the equatorial plane in the direction opposite that of rolling of the tire.
The state of the art has not fully solved the problem of producing a tire provided with a tread pattern that may regardlessly be of the symmetrical, asymmetrical or directional type and at the same time be capable of minimizing and rendering even the wear of the tread band while the tire is rolling, and of improving and rendering uniform the longitudinal and transversal rigidity so as to ensure an effective stability both on the straight and in bends on dry roads, all of which without impairing the characteristics of low rolling resistance, of sufficiently quiet operation and of good driveability including on terrains that are not dry, for example on roads that are wet or covered in snow.
The tire described in patent WO 98/25776 has transversal grooves extending with continuity and great density from the shoulder zones to the zone straddling the equatorial plane, thereby giving rise to a plurality of narrow and elongated blocks, arranged obliquely to the equatorial plane and excessively deformable. In this way, uneven wear may occur of the elastomeric material delimiting these grooves, particularly under conditions where the tire is rolling on a straight, dry road or at any rate under conditions where the stresses acting along the equatorial plane are considerably greater than those acting in a direction perpendicular to this plane.